This invention relates to thermosetting resins and more particularly to thermosetting cyanate esters. Still more particularly, this invention relates to the use of urea compounds as latent cure accelerators for cyanate esters, and to thermosetting cyanate ester formulations comprising cyanate esters and urea compounds.
Cyanate esters are well-known in the art and widely used in formulating adhesives, binders, coatings and impregnants. Such formulations may also include oligomeric compounds with reactive cyanate ester functionality as well as a variety of other coreactants such as epoxy resins in order to reduce costs and to modify properties such as toughness, moisture sensitivity and thermal behavior in the resulting thermoset materials.
Cyanate esters may generally be cured merely by heating. Catalysts used to promote curing under milder conditions have included Lewis acids such as aluminum chloride, ferric chloride and the like, mineral acids such as hydrochloric acid, salts such as sodium acetate, potassium thiocyanate and the like, phenolic compounds and bases such as sodium methoxide, pyridine, triethyl amine and the like. Metal chelates such as copper, zinc or ferric acetylacetonates have been reported as being capable of promoting a smooth, controllable cure rate at moderate temperatures. Such catalysts are said to be generally less moisture sensitive, and possibly less hazardous than many of the catalyst systems available for cyanate esters.
Many of the prior art catalysts are highly active and may even promote rapid curing at room temperature in many cyanate ester formulations. The storage stability of cyanate ester materials and formulations based on such catalysts may therefore be brief, making the formulations difficult to use for many applications by requiring storage conditions that may be difficult or impractical to achieve. The more stable cyanate ester formulations, those based on the less-active prior art catalysts, may be more difficult to cure adequately even when extended cure cycles are used. Extended cure times, particularly at elevated temperatures, increase the cost of production and may cause damage to substrates as well as to other components of the formulation. In addition, insufficient cure levels tend to result in brittle materials having an increased sensitivity toward moisture. Cyanate ester cure accelerators having little or no catalyst activity at or near room temperature and a high degree of activity at moderately elevated temperatures are thus needed. Such accelerators are termed latent cure accelerators, and may be used to provide storage-stable cyanate ester formulations that are rapidly and completely cured at moderate temperatures.
Some of the presently available catalysts exhibit a degree of latent curing character or latency when used in combination with some cyanate resins. However, such catalysts are few in number. The uses of cyanate ester formulations by the coatings, adhesives and laminating arts encompass a great variety of applications. The curing conditions required by these applications will vary widely, and the latent curing behavior needed for some applications may be measured in hours, while others may require stability for days or even weeks at room temperature. Moreover, the residues characteristic of some catalysts may not be acceptable for particular applications and end uses. Thus there is a continuing need for a greater variety of cure catalysts and latent cure accelerators, in order to allow the resin formulator to modify the curing behavior and storage characteristics of cyanate ester-based resin formulations, thereby becoming better able to meet the demands of these industries.